1. Field of the Invention
The present invention relates to a rotary type electrostatic spray coating device. More particularly, the present invention relates to an improvement of side spray coating unit used in such a spray coating device. The spray coating device according to the present invention can be effectively used for spray coating automotive bodies and other work pieces with different paints.
2. Description of the Related Art
As is well-known in the art, a variety of electrostatic spray coating processes and devices have been widely used in the spray coating of different size work pieces. In particular, a rotary type electrostatic spray coating process and device has been used due to the excellent coating efficiency and other effects thereof. Generally, the prior art rotary type electrostatic spray coating device, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-115652, has a structure wherein a rotary shaft with a fixed spray head such as a bell cup or mini-bell cup is rotatably supported in a metallic air motor housing, the rotary shaft is rotated at a high speed by an air motor, while a negative voltage is continuously applied thereto from a high voltage generator, a paint is fed onto a cup-shaped inner wall of the spray head, and thus a negatively charged paint mist is emitted from the spray head and sprayed onto a surface of the work piece to be spray coated, such as the body of a motor car. A similar rotary type electrostatic spray coating device is described in U.S. Pat. No. 4,350,304 issued Sept. 21, 1982.
In addition, many improved rotary type electrostatic spray coating devices have been disclosed in Japanese and U.S. patents and applications. For example, Japanese Unexamined Patent Publication (Kokai) No. 56-139164 discloses an improvement of a reciprocating machine in the spray coating device. To ensure an uniform thickness of the resulting paint coating, Kokai '164 teaches to incline a spray coating equipment movable on the reciprocating machine so that a spray head thereof faces only a work piece to be coated. Japanese Unexamined Patent Publication (Kokai) No. 57-165064 discloses an improvement of the spray coating of an outer plate of an automotive body. Kokai '064 teaches to adjust a direction of the spray head in accordance with the portion of the outer plate to be coated, thus obtaining a uniform thickness of the resulting coating.
The above two Japanese Publications concern an improvement of the uniformity of the paint coating. Japanese Unexamined Patent Publication (Kokai) No. 57-194072 discloses that, if a discharge of the charged paint is temporarily stopped and a differential potential of the work piece to be coated with regard to the charged paint is reduced when the spray coating equipment reaches a location where coating is not required, an error in operation of the high voltage shut-off device therein can be prevented. Japanese Unexamined Patent Publication (Kokai) No. 57-204265 teaches that, to prevent coating defects such as electrostatic craters and the so-called "flowering phenomenon" due to electrostatic charges, a dark electric current passed through a spray coating device to a work piece should be adjusted to a value lower than a predetermined value. Japanese Unexamined Patent Publication (Kokai) No. 58-174269 teaches to provide a high-speed and low-speed flow of air in a spray booth to increase a paint coating efficiency on a work, to prevent flying and adhesion of the spray dust, and reduce the amounts of air introduced into and discharged from the spray booth. Japanese Unexamined Utility Model Publication (Kokai) No. 61-139775 teaches to install spray guns on the side walls of the spray booth and to form air curtains along said side walls to prevent flying and deposition of not-used paint mists on said side walls and guns.
There are many U.S. patents disclosing improved electrostatic spray coating devices. U.S. Pat. No. 3,691,991 discloses a spray coating apparatus which includes a removable and replaceable supply container for the coating material. The apparatus of this U.S. patent has many advantages, such as a simplification of the adjustment of the apparatus, switching over of the high voltage supply at a control desk, and quick cleaning of the injector and a quick change-over from one color to another color. U.S. Pat. No. 4,232,055 concerns an improved electrostatic spraying system capable of automatically painting work pieces with different colored electrically conductive paints. According to this system, it is possible to selectively spray a plurality of paints with different colors. U.S. Pat. No. 4,676,189 of the present inventors, issued later than the filing date of Japanese patent application No. 61-309262 (basis of the instant application), concerns an attachment construction of a hosepipe of an electrostatic spray coating apparatus. The functioning of the apparatus can be tested before installation at a work location.
Referring again to Japanese Kokai No. 57-204265, FIG. 1 thereof (reproduced herein also as FIG. 1) illustrates a typical rotary-type electrostatic spray coating device for automotive bodies. In the illustrated spray coating line, automotive bodies 1 each having door portions 2 and protruding portions 3 are continuously conveyed in the direction indicated by an arrow A. Both side portions of the automotive body 1 are spray coated, during this passage through the line, in sequence by a pair of low-positioned side spray heads 4 and by a pair of high-positioned side spray heads 5. The body 1 is then guided under a pair of top spray heads 6 installed on a gate-type reciprocator 7, and thus the top portion of the body 1 is spray coated. Although FIG. 1 shows only two pairs of side spray heads, conventionally three pairs of the side spray heads are disposed in the spray coating line to attain a satisfactory paint coating efficiency. In addition, recently, as illustrated in FIG. 2, four pairs of side spray heads are frequently disposed in the spray coating line to further increase the paints coating efficiency and to widen the coating area. The spray coating booth shown in FIG. 2 has two pairs of low-positioned side spray heads 4 and 14, two pairs of high-positioned side spray heads 5 and 15, and a pair of top spray heads 6.
There is a tendency to increase the number of side spray coating units in the spray booth, as can be appreciated from the above descriptions concerning the prior art spray coating devices, to increase a paint coating efficiency thereof. However, this increase in the number of side spray coating units, particularly the side spray heads, causes a problem in that the paint mist from the side spray heads adheres to other side spray heads during the spraying operation. This problem arises due to the switching on and off of the high voltage generators for the side spray heads, and will be now described with reference to FIG. 2, in which four pairs of side array heads are disposed along a line conveying automotive bodies. For example, when all of the side spray heads 4, 14, 5 and 15 are simultaneously used for spray coating, only a negligible amount of the paint mist is deposited on each spray head, since all spray heads are maintained under the same high potential during spray coating. However, if one of said spray heads, for example, spray heads 14, are not operated and are washed with a thinner for color changing, a large amount of the paint mist from the remaining spray heads 4, 5, and 15 being operated will adhere to the spray heads 14, because a difference in potential is generated between the heads 14 and heads 4, 5, and 15 as a result of a removal of the high voltage from the heads 14 during the color change washing. To avoid this adhesion of the paint mist, generally two adjacent side spray heads are disposed at a large distance or bell pitch of about 1.5 to 1.8 mm. The increase of the distance between the subsequent spray heads also causes an increase of the length of the spray booth and a rise in the initial costs and running costs because of a corresponding extension of the air conditioning space.
In addition, another problem arises of overlapping of the coating patterns of the paint from two adjacent spray heads, such as two successive side spray heads and two adjacent side and top spray heads. When there is a large distance or bell pitch between these two spray heads or the conveyor of automotive bodies is driven at a low speed, i.e., when there is a large time lag from the end of the spray coating to the start of the next spray coating, a good flatness of the coating surface can not be obtained on the spray coated automotive bodies due to a complete drying of the particles of the previously coated paints.